ANY post-1986 full auto can't be owned, unless you are a police dept, government (like the army), or you are a class III FFL holder (a VERY hard license to obtain). Any pre-1986 full auto can be owned by most US citizens with some extra paperwork and a $200 tax stamp.
Many full auto weapons can be modified to be semi-auto only, and then they are much easier and cheaper to own, regardless of the year of manufacture. These are the general rules for owning a firearm. State laws can also factor in...
Now, how do you actually find one for sale? Well, i think a guy on the weaponeer website had/has one, but don't expect to go in there and just start asking questions. Those guys are a bit paranoid and will assume any FA questions from a new member are secretly coming from an ATF officer trying to trick them. They really are a paranoid bunch, but can build guns from scratch and are filled with helpful building and modding knowledge...